Boaters rely on sonar to navigate their boats through perils. Underwater rocks could sink a boat. If boats run aground in shallow water, the people onboard could be stranded. Sonar is used to detect explosive mines that are meant to sink and destroy ships. Sonar is also used in seafloor mapping and other geological and biological studies. Additionally, people use sonar to look for fish, shipwrecks, and other features of interest. Depending on which use is most important, a boater will choose the type of sonar that they want to install on their boat.
For example, a typical down-looking echosounder mounts on the bottom of a boat's hull and uses a narrow acoustic beam to scan the seafloor. By calculating the time for the sound to return, the echosounder can reveal the depth of the water as well as the presence and location of fish. In fact, many echosounders are sold as fishfinders.
However, where navigation and hazard avoidance is the goal, forward-looking sonar is used. Forward-looking sonar scans ahead of the vessel. These devices show the seafloor and obstacles ahead of the vessel. With some forward-looking devices, a single ping can generate a three-dimensional map of the area in front of the vessel.
To search the greatest area, side scanning sonar is used. As the name suggests, side-imaging units scan the sides of the boat and can produce detailed historical pictures out from the side of a boat. While side scanning sonar is not optimum for fish-finding or navigation, the breadth of coverage is good for finding reefs, shipwrecks, and other places where fish tend to congregate.
Unfortunately, once a boater decides to use an echosounder, forward-looking sonar, or a side scanner, the use of one compromises the ability to use any other. Not only are boaters understandably reluctant to drill the additional holes through the hulls of their boats, operation of the units actually interferes with one another. The sonic signal from an echosounder, for example, will overlap with, and thus muddy, the signal from a side scanner. So, if a boater chooses to use a fishfinder, then he must check if he has false alarms in or from forward-looking sonar due to the signals interference, which is quite a complicated task for the ordinary user who is not a sonar engineer.